Thank you for posting. There's no way to change the operating system of your hosting account. What you would need to do is purchase a new plan in the operating system you want to have and move your site to the new plan. If the old plan is within our refund policy, you could cancel it and reach out to support for a refund.
After you purchase your new plan, just make a site backup + restore it to your new plan.
Be sure you can bring up your site under your new plan, before retiring your old plan.